The present invention relates generally to an exposure system for photographic copying machines, such as copying machines used for reproducing original images of a film strip of negatives onto a photographic copying material (e.g., photographic paper).
In photographic copying machines commonly used today, copies are usually made from originals in such a way that the originals being copied, for example in the form of picture frames on a film strip of negatives, are measured and then exposed with the required mount of copying light onto photographic copying material, such as photographic paper. During the exposure, the image field being exposed onto the photographic paper is reproduced onto the paper by means of an exposure system and imaging optics. In order to obtain a reproduction that is as similar to the original as possible, the light coming from a light source, for example from a bulb, is "dyed." The light is dyed by means of optical color filters, which are inserted in the path of the light beams. The dyed light is then guided to the original (e.g., to a negative), which is supposed to be exposed onto the photographic paper, and illuminates the original. Imaging optics are also located in the path of light for the original, whereby the exposed original is then reproduced onto the photographic paper.
An exposure system of this kind is described, for example, in DE 32 26 408. A bulb housing with a light source and an ellipsoid reflector for better use of the light emanating from the light source is provided in the exposure system described in the aforementioned patent, whereby the bulb coil of the light source is positioned at the first focal point of the ellipsoid reflector. The light emanating from the bulb housing is dyed as it proceeds along its course by means of a device, which has adjustable color filters in the beam path and a diaphragm with a diaphragm aperture. This device is positioned at a specific distance in front of or behind the second focal point of the ellipsoid reflector in such a way that the interior region of the diaphragm aperture is located in the shadow of the light source. The distance is a function of the diameter of the diaphragm aperture, of the small semi-axis of the ellipse that forms the basis for the ellipsoid reflector, and of the focal length of this ellipse.
The color filters of this device in the exposure system described above have three batteries of filters. Each battery of filters, when swung into place, covers roughly a third of the cross section of the cone of light entering the diaphragm aperture, whereby the battery of filters is swung into the light path by mechanical means.
But such batteries of filters have certain disadvantages. For example, when such filters are swung into place mechanically, it cannot be assured that the individual glass filters of the battery of filters will penetrate to precisely the same depth in the cone of light, i.e., that they cover at each time a precisely identical mount of the cross section of the cone of light passing through the diaphragm aperture. This is more or less unavoidable when the glass filters are swung into place mechanically. As a result, this mechanically-related uneven permeation of the glass filters can lead to inhomogeneities of the intensity distribution on the negative or to an intensity distribution that deviates from the desired intensity distribution. Furthermore, when the glass filters are arranged in the light beam path, it is also possible for the edges of the filters or their shapes to produce more or less contrast-rich structures in their intensity distribution. The result of this intensity distribution that deviates from the desired intensity distribution on the negative is, in turn, a concomitant poorer color reproduction of the contents of the negative onto the photographic paper.
This is even more so the case since the transmission characteristics of the optical glass filters are never exactly identical, as a result of manufacturing idiosyncracies, for the given color in the three batteries of filters, so that when the glass filters are placed in the path of the light beam, then certain parts of the cone of light penetrating the diaphragm aperture are dyed to a greater or lesser extent. Furthermore, the transmission characteristics of such glass filters are not even (i.e., homogeneous) across the entire surface of the glass filters and also display a certain structure (so-called density structure). This structure of the filters can be present on the original (negative) and is, as a result, also reproduced by the imaging optics onto the copying material (photographic paper). The deeper the given glass filter is moved into the path of the light beam, the more the shape and irregularities can become noticeable on the transmission characteristics of the glass filter. The result can be an inhomogeneous intensity distribution or an intensity distribution that deviates from the desired intensity distribution of the light of the given color onto the negative as well as a reproduction of the structure of the glass filter. In this way, the true to color reproduction of the content of the negative onto the photographic paper can be influenced more or less negatively because the imaging optics only reproduce the negative onto the photographic paper.